1. Field of the Invention
This invention relates to a thick film thermal head and a method of manufacturing the same.
2. Description of the Related Art
As the thermal head used in various image forming apparatuses, there have been known a thin film thermal head and a thick film thermal head. The former is formed by the use of thin film forming technique and the latter is formed by the use of technique other than the thin film forming technique. When perforating a heat-sensitive stencil material to make a stencil for a stencil printer by the use of such a thermal head, it is required that adjacent perforations are clearly separated in order to obtain a high printing quality. Further, in order to make feasible stencil printing in a large size, e.g., A2 size or larger sizes, it is required to make a thermal head in a large size. Further, since the manufacturing process and the manufacturing cost of the thermal head occupy a large part of the manufacturing process and the manufacturing cost of the stencil making apparatus for a stencil printer, there has been a demand for a thermal head which can be easily manufactured at low cost.
Generally, the thin film thermal head is manufactured by a high-level process using semiconductor manufacturing technology and expensive apparatuses such as a sputtering apparatus or a vacuum deposition apparatus, and accordingly, the manufacturing process of the thin film thermal head is complicated and the manufacturing cost of the thin film thermal head is high though the pattern and the dimensions of the electrodes and the resistance heater elements can be finely controlled. Further, the length of the thin film thermal head which can be manufactured by the use of an existing apparatus is 8 to 12 inches at the longest. To the contrast, the thick film thermal head can be produced, for instance, by screen printing, and can be easily produced at low cost and can be easily produced in a large size. However, it is very difficult to accurately control the dimensions of the electrodes and the resistance heater elements (especially the dimension of the resistance heater elements in the direction of width of the thermal head) of the thick film thermal head. Thus the thin film thermal head is advantageous over the thick film thermal head in some points and the latter is advantageous over the former in other points.
The thick film thermal head has been generally used in a thermal recording system and a ribbon transfer printing system. The thick film thermal head generally comprises an electrical insulating substrate such as of ceramic, a plurality of stripe electrodes formed on the substrate and a linear resistance heater strip formed on the electrodes. In this thick film thermal head, the resistance heater strip extends across the electrodes and the parts of the resistance heater strip between the electrodes form resistance heater elements. That is, when power is supplied to the electrodes, the resistance heater strip generates heat at the parts between the electrodes. Since the heater strip is in contact with the electrodes at the lower surface thereof, heat is generated from the lower surface of each resistance heater element and propagates the resistance heater element to the upper surface thereof where the resistance heater element is brought into contact with a recording medium. In this thermal head, heat generated from the lower surface of each resistance heater element spreads in various directions while it propagates the resistance heater element to the upper surface thereof, and each pixel of the image formed by the thermal head becomes larger than the heater element, which results in pixels contiguous to each other. In the thermal recording system and the ribbon transfer printing system, this is advantageous in that pixels (dots) can be formed in a state where the pixels are continuous to an extent proper to obtain a high quality image.
However, when the thick film thermal head is used for making a stencil as it is, each of the perforations becomes too large and the perforations cannot be discrete since the heat generated from the lower surface of each of the resistance heater elements spreads over a wide area while the heat propagates to the upper surface of the heat element, and at the same time, it takes a long time for the temperature of the surface of each heater element to reach a perforating temperature, which results in poor response of the thermal head. When the perforations are not discrete and are connected to each other, an excessive amount of ink is transferred to the printing paper through the stencil, which results in offset and/or strike through. Further, in the case of a stencil printer, ink is apt to spread when transferred to the printing paper through the perforations of the stencil and is apt to form printing dots larger than the perforations of the stencil. Accordingly, the perforations of the stencil should be smaller by an amount corresponding to spread of the ink and should be discrete from each other. From this viewpoint, the aforesaid thermal head where heat is generated from the lower surface of the resistance heater elements is not suitable for making a stencil.
In a thick film thermal head having a linear array of resistance heater elements extending in a main scanning direction (in the direction of width of a stencil), though the size of the perforations in the main scanning direction can be reduced by narrowing the intervals at which the electrodes are arranged, it is difficult to reduce the size of the perforations in the sub-scanning direction (the direction in which the stencil is conveyed) due to difficulties in narrowing the width of the resistance heater strip(e.g., to not larger than 100 .mu.m).
That is, conventionally, the thick film thermal head is formed by coating resistance heater paste 30 by silk screening on electrodes 50 formed on an electrical insulating substrate 100 as shown in FIG. 15. Though the resistance heater paste 30 forms a narrow protrusion as shown by chained line immediately after coating, it is flattened in the sub-scanning direction with lapse of time as indicated at 31. This phenomenon occurs because the resistance heater paste 30 is flowable and there is provided no member for limiting spread of the paste, and makes it difficult to form a narrow resistance heater.
Also in the thermal recording system and the ribbon transfer printing system, there has been a problem that it is very difficult to improve printing resolution due to difficulties in narrowing the width of the resistance heater strip (e.g., to not larger than 100 .mu.m). Further, as the thermal head is repeatedly driven, heat generated from the resistance heater elements accumulates in the thermal head, which results in a problem that the thermal response of each heater element deteriorates or control of the temperature of each heater element becomes difficult. The delay from the time the heat is generated at the lower surface of the heater elements to the time the heat is transferred to the upper surface of the same further enhance deterioration of the thermal response of the heater elements.
From the viewpoint of making smaller the perforations formed in the stencil material and making higher the printing resolution, the thin film thermal head is advantageous over the thick filmthermal head. In the thin filmthermal head, the width and/or shape of the heater elements can be controlled much more finely than in the thick film thermal head due to the difference in manufacturing process. However, the thin film thermal head is disadvantageous in that it is expensive and is difficult to produce in a large size as described above. That is, since the thin film thermal head is manufactured by the use of semiconductor manufacturing apparatuses which are generally for making integral circuits and the like and are not able to produce a large size thermal head by one step. Accordingly, a large size thin film thermal head must be produced by incorporating a plurality of small thermal head segments, which gives rise to a problem that heat generation becomes unsatisfactory at junctions between the segments, which can result in white stripes on prints. Further, difference in heat generating characteristic between the small thermal head segments can result in fluctuation in the printing density and can adversely affect the image quality of the prints. Though these problems may be overcome by carefully joining the thermal head segments, this approach deteriorates the yield of the thermal head and further adds to the manufacturing cost of the thermal head.
Further, since the thin film thermal head is formed of thin films, the resistance heater elements are small in volume and heat capacity. Accordingly, in order to ensure an amount of heat sufficient to properly perforate the stencil material, an excessively large amount of power must be supplied to the resistance heater elements and accordingly the resistance heater elements are apt to be deteriorated or damaged. Therefore, use of the thin film thermal head in stencil making is limited. For example, the thin film thermal head can be only used for stencil materials comprising a heat-sensitive film whose thickness and melting point are in predetermined ranges. When the thin film thermal head is used for perforating a stencil material whose thickness and melting point are not in the predetermined ranges, the resistance heater elements must be driven under excessive load and the resistance heater elements are more apt to be deteriorated or damaged, which results in deterioration in reliability and/or durability of the thermal head.
The stencil material for stencil printing generally comprises a laminate of a support sheet such as Japanese paper or gauze and a heat-sensitive film, or a heat-sensitive film alone. The stencil material comprising a heat-sensitive film alone is advantageous in that ink transferred to the printing paper through the perforations in the stencil is not interfered with a support sheet and a clear printed image can be obtained.
However, without a support sheet, the stencil material is not sufficient in mechanical strength and apt to be stretched or deformed during conveyance or the like. Accordingly, in the stencil material without a support sheet, the heat-sensitive film must be larger in thickness than in the stencil material with a support sheet. However, it is very difficult to surely perforate such a thick heat-sensitive film with the thin film thermal head which is limited in heat capacity.
Though a ceramic substrate has been conventionally employed in both the thick film thermal head and the thin film thermal head, the ceramic substrate is disadvantageous in that it generally requires a complicated manufacturing process, it is high in material cost and manufacturing cost, and it is difficult to form a highly smooth large surface.
Further, in the conventional thick film thermal head, the resistance heater strip is in the form of a protrusion on a substrate. This is disadvantageous in that paper grounds or resin grounds is peeled off the stencil material by the protruding resistance heater strip when the stencil material is moved relative to the thermal head during stencil making. The paper grounds or the resin grounds adheres to the surface of the protruding resistance heater strip and adversely affects stencil making, e.g., prevents the resistance heater strip from being brought into a close contact with the stencil material and causes the resistance heater strip to fail in perforating the stencil material.
As can be understood from the description above, though the conventional thick film thermal head is advantageous in that it can be easily manufactured at low cost and can be manufactured in a large size, it is very difficult to more finely perforate the stencil material and to suppress formation of connected perforations, or to print on a heat-sensitive recording medium or a printing paper at higher resolution, and to improve response of each resistance heater element. Further, the conventional thick film thermal head is disadvantageous in that paper grounds or resin grounds is apt to be generated and adversely affects stencil making or printing.